Explosion proof enclosures are commonly used in hazardous locations in order to contain explosions that may occur within the enclosure and prevent sparks occurring within the enclosure from igniting vapors, gases, or other materials in the area surrounding the enclosure. Hazardous locations may include, for example, aircraft hangars, gasoline stations, paint finishing locations, agricultural areas, etc.
The National Electric Code (NEC®) defines classes and divisions of hazardous locations, as well as requirements for explosion proof enclosures used in such locations. For example, a Class I hazardous location or area is one in which flammable gases or vapors are/could become present in concentrations suitable to produce explosive and/or ignitable mixtures. A typical class I area is a petroleum processing facility, for example. Within Class I, a Division 1 area or location includes: one where the atmosphere of the area is expected to contain explosive mixtures of gases, vapors or liquids during normal working operations; one where ignitable concentrations frequently exist because of repair or maintenance operations; or one where there is release of ignitable concentrations of gases or vapors due to equipment breakdown, while at the same time causing electrical equipment failure. A Class I, Division 2 hazardous location includes: one where flammable liquids or gases are handled, but not expected to be in explosive concentrations, with the possibility of explosive concentrations resulting from an accidental rupture or other unexpected incident; one where ignitable gases or vapors are normally prevented from accumulating by positive mechanical ventilation, but could exist in ignitable quantities if there is a failure in the ventilation system; and areas adjacent to Class I, Division 1 locations where it is possible for ignitable concentrations of gas/vapors to enter the area due to lack of proper ventilation.
Explosion proof enclosures often have conduits leading to them. Such conduits may carry cable or wiring, for example, to power and/or control machinery inside the enclosure, such as a motor or a compressor.
The features of the conduit and how they connect to the explosion proof enclosure are highly regulated. For example the NEC provides that in Class I, Division 1 locations, all conduits generally must be rigid metal or steel IMC with at least five fully tapered threads tightly engaged in the enclosure. In some applications, conduits are fortified with one or more corrosion resistant coatings.
Conduit systems are generally not airtight, such that the type and concentration of gases outside the conduit can also be present inside the conduit. Thus, conduits are provided with seals to prevent explosions from spreading through conduit systems. Seals create a physical barrier that minimizes the passage of gases travelling through the conduit, and also limit the passage of vapors between hazardous and non-hazardous locations. Seals can also serve to prevent pressure piling, i.e., the buildup of pressure inside a conduit caused by precompression as the explosion travels through the conduit.
Conduit system seals are usually provided in a sealing fitting. An example of an industry standard sealing fitting 30 is depicted in FIG. 2. The NEC provides that sealing fittings are required at each entrance to an enclosure housing an arcing or sparking device when used in a Class I, Division 1 and 2 hazardous locations, and at each entrance of two inch size or larger to an enclosure or fitting housing terminals, splices or taps when used in Class I, Division 1 hazardous locations. In these scenarios, the sealing fitting must be as close as practicable to the enclosure and in no case more than 18 inches from the enclosure. In addition, the NEC provides for installing sealing fittings in conduit systems when leaving Class I, Division 1 or 2 hazardous locations, and in cable systems when the cables either do not have a gas/vaportight continuous sheath or are capable of transmitting gases or vapors through the cable core when those cables leave Class I, Division 1 or 2 hazardous locations.
As mentioned above, typical conduit systems are not airtight. Therefore, changes in temperature or barometric pressure can cause a “breathing” effect in which air enters the conduit system or enclosure. Once air enters a conduit, for example, moisture in the air can condense. If the conduit runs non-horizontally, the condensed water can collect at the base of the conduit, potentially causing equipment shorts or grounds. In these situations, sealing fittings that include a drainage feature can be used to provide an explosion proof path for water to exit while still sealing the conduit.
The seals in sealing fittings are inspected to determine if the seal is compromised and should be reinforced or replaced. Typically, the inspection has to be performed visually through a port provided in the sealing fitting. Such inspection will often fail to reveal sealant imperfections or degradations occurring outside the small field of view (i.e., the sealant directly under the plug) available during a visual inspection, which would otherwise alert the technician to a potentially hazardous condition that should be remedied. In addition, seals in sealing fittings are often prepared by electricians, who may not have extensive training in how to properly and sufficiently introduce sealant to the sealing fitting.
There is a need to improve sealant inspection and verification in sealing fittings used in or near hazardous locations.